It is often a primary goal of wastewater treatment to reduce phosphorous and nitrate levels in the treated wastewater, allowing the treated wastewater to be discharged to the environment. Many governmental bodies have introduced strict wastewater treatment plant effluent phosphorous and nitrate level requirements, which requirements can be quite difficult to reach with traditional treatment techniques.
Phosphorous can be introduced into a water system via human waste, agricultural and urban runoff, fertilizers and certain detergents. Nitrate is typically introduced through the oxidation of ammonia in biological wastewater treatment processes. High levels of biologically available phosphorous and nitrate in treated wastewater that is discharged into the environment can lead to eutrophication of lakes, rivers and estuaries. Certain specific wastewater treatment techniques have been developed which can minimize the phosphorous and nitrate content of treated wastewater. Two commonly employed methods of phosphorous reduction are chemically enhanced primary treatment (CEPT) and biological phosphorous removal (BPR). Nitrate reductions are typically accomplished through biological action in non-aerated zones of bioreactors. In these non-aerated zones, nitrate is used as an electron acceptor by microorganisms. Nitrate may be used in place of oxygen under non-aerobic conditions. As the micoorganisms consume soluble organic matter in the non-aerated zones, they reduce soluble nitrate to nitrogen gas. The nitrogen gas is then released to the atmosphere. In addition to nitrate reduction in the plant effluent, nitrate reductions are also typically needed in BPR systems. BPR systems require anaerobic zones to function. A non-aerated zone can be either anerobic and anoxic. The difference being in the nitrate levels present in the zone. A non-aerated zone with little to no nitrate can be considered anaerobic. A non-aerated zone with significant levels of nitrate is considered to be anoxic. Since BPR requires anaerobic zones, the removal of nitrate (if present) is a key part of the process.
CEPT is a process in which chemicals, typically metal salts and/or polymers in the form of organic polyelectrolytes are added to wastewater. Often, the chemicals are added after screening and sometimes after de-gritting. The chemicals may be added in the primary sedimentation basins of a wastewater treatment plant or in a dedicated CEPT tank. The chemicals utilized in CEPT are the same as are commonly added in potable water treatment, for example, ferric chloride, ferrous chloride, ferrous sulfate, polyaluminum chloride (PACL), and aluminum sulfate (alum).
The chemicals added to the wastewater cause organic matter, including phosphorous (PO4−3), to precipitate and clump together via the processes of coagulation and flocculation. The particle aggregates, or flocs, settle faster than occurs in an untreated primary sedimentation basin, thereby enhancing primary wastewater treatment efficiency as measured by the removal of solids, organic matter and nutrients, including phosphorous and nitrogen, from the wastewater. Metal salts also promote the removal of phosphorous by facilitating chemical precipitation of soluble phosphorous in solution, where phosphorous bound within the precipitate settles with the sludge and can be removed along with the sludge.
The dosage of the chemical added in a CEPT facility can be varied to accomplish the removal of either a portion or substantially all of the soluble phosphorous content of the wastewater. This may be done in a manner that will provide responsive and reliable control of the discharge of phosphorus from a wastewater treatment facility.
Another advantage of CEPT systems is that they are capable of significantly reducing the TSS and BOD load to a downstream biological treatment system. This allows the biological system to be either sized with smaller tankage and aeration systems, thus reducing capital and operating costs, or allows increasing the treatment capacity of existing biological systems.
Three primary drawbacks are associated with CEPT. First, the use of chemicals in primary treatment can substantially increase the cost of primary treatment. Second, the amount of chemicals necessary to provide coagulation, flocculation and precipitation sufficient to meet effluent phosphorous goals without substantial secondary treatment will result in increased primary sludge production. According to Baur, U.S. Pat. No. 6,387,264, the use of “metal salts . . . may not only be expensive, but it also produces a chemical sludge that tends to be “fluffy,” thereby increasing overall sludge volume.” A higher overall sludge volume makes it harder to remove water from the sludge, which also makes the sludge harder to dispose of, thereby increasing disposal costs. Third, the increased BOD removal associated with CEPT reduces the amount of BOD available in the secondary system that can be used to remove nitrate from solution. This can increase nitrate levels in the effluent and hinder the development of a BPR system. Other drawbacks of CEPT are the need for extra storage space for the chemicals, the chemicals used in the CEPT process, such as ferric chloride, tend to be highly corrosive, and having such chemicals onsite at the wastewater treatment facility increases safety concerns and liability exposure for the plant.
BPR is an alternative phosphorous removal technique, which relies upon the actions of specific microorganisms to minimize the phosphorous content of wastewater treatment plant effluent. Many types of microorganisms contribute to removing phosphorous in the BPR process. As a group, the key microorganisms are known as phosphorous accumulating organisms (PAOs). Many configurations of BPR treatment plants are known in the art. All BPR treatment methods and facilities, however, share the following common features: the wastewater is treated in an anaerobic zone having organisms that release phosphorous and consume volatile fatty acids to create energy stores in the form of polyhydroxylalkonates (PHAs). The wastewater is treated in one or more aerobic zones having organisms which also metabolize stored PHAs and uptake phosphorous. When there is also nitrate in the wastewater, the wastewater may also be treated in one or more anoxic zones having organisms which reduce nitrate and metabolize stored PHAs and uptake phosphorous. Different configurations of BPR facilities known in the art will vary the number and order of each of the anaerobic, anoxic and aerobic zones. In addition, the retention time and/or flow rate through each zone can be varied along with specific effluent recycling patterns or treatment paths. The net result of treatment of wastewater with a BPR system is a reduction in the effluent phosphorous, and sometimes nitrate, to acceptable levels. All known BPR methods rely on the use of volatile fatty acids (VFAs), typically acetic acid, as an energy source for the PAOs which uptake phosphorous from the wastewater. BPR can not proceed without sufficient VFAs or with too much nitrate in the wastewater.
CEPT is not normally compatible with BPR or nitrate removal since, in addition to phosphate, significant particulate/colloidal biological oxygen demand (BOD) is removed from the wastewater in CEPT. Influent to a BPR process which is low in BOD may not have sufficient VFAs to support an effective PAO population or sufficient BOD to allow nitrate to be removed to the low levels required in BPR. Thus, use of CEPT followed by a downstream BPR or nitrate removal operation is contrary to conventional nutrient removal designs.
Since the PAOs which drive BPR rely on VFAs for energy, certain methods have been developed to increase the VFA content of the BPR influent. Acetic acid can be added directly to the BPR influent, although this is expensive. Alternatively, primary sludge from the primary treatment stage of wastewater treatment can be fermented to create VFAs for later use in BPR or nitrate removal. The fundamental biological processes of fermentation are hydrolysis and acetogenesis. The products of hydrolysis are soluble organic acids which contain carbon. Acetogenesis converts those acids to volatile fatty acids.
As noted above, the two techniques of phosphorous removal, CEPT and BPR, are considered to be mutually exclusive. For example, Baur teaches a fermentation and thickening process that generates a supernatant having a concentration of VFAs suitable for driving a BPR process. Baur, however, presents the fermentation/BPR process as a superior alternative to CEPT with metal salts.
Similarly, Husain, U.S. Pat. No. 6,485,645, teaches a BPR method and apparatus featuring the use of a membrane filter to separate an effluent lean in phosphorous from a liquid rich in rejected solids and organisms in the BPR anoxic zone. According to Husain, CEPT, as an alternative treatment method is less desirable than BPR since, “chemical precipitation methods . . . result in high chemical costs, high sludge production and a high level of metallic impurities in the sludge.”
Thus, there exists a need in the art for a wastewater treatment method which combines the advantages of CEPT with the production of sufficient VFAs/BOD to support secondary BPR treatment.
In a similar manner, practicing CEPT with a downstream biological nitrate removal system is normally considered to be counter-productive. CEPT will remove a greater fraction of the biodegradable organic material needed to reduce nitrate. Similarly to the situation with phosphorus, there also exists a need to combine the advantages of CEPT with the supply of sufficient BOD to support effective nitrate reduction.